Diamond provides a wide and useful range of properties, including extreme mechanical hardness, low coefficient of thermal expansion, high chemical inertness and wear resistance, low friction, and high thermal conductivity. Generally, diamond is also electrically insulating and optically transparent from the ultra-violet (UV) to the far infrared (IR), with the only absorption occurring from carbon-carbon bands that range from about 2.5 μm to 6 μm. Given their properties, diamond can be utilized in many diverse applications in industry, however its overall utilization has long been hampered by the comparative scarcity of natural diamond. In turn, there has been a long-running quest for processes to synthesize diamond in the laboratory.
Synthetic diamonds are currently produced by a variety of methods. One such method involves a process referred to as chemical vapor deposition (CVD). CVD diamond has only been commercially synthesized for the last 15 years. This diamond growing method involves providing a hydrocarbon gas (typically methane) in an excess of atomic hydrogen. Generally, a gas-phase chemical reaction occurs above a solid surface, which causes deposition onto that surface. All CVD techniques for producing diamond films require a means of activating the gas-phase carbon-containing precursor molecules. This generally involves thermal (e.g., hot filament) or plasma (e.g., D.C., R.F., or microwave) activation, or the use of a combustion flame (oxyacetylene or plasma torches). Two of the more popular experimental methods include the use of a hot filament reactor, and the use of a microwave plasma enhanced reactor. While each method differs in regards to activation, they all share similar aspects otherwise. For example, growth of CVD diamond (rather than deposition of other, less well-defined, forms of carbon) normally requires that the substrate be maintained at a temperature in the range of 1000-1400 K, and that the precursor gas be diluted in an excess of hydrogen (typical CH4 mixing ratio ˜1%-12% in volume).
CVD diamond grows in a two-dimensional manner, layer by layer, and it is therefore possible to build up a bulk diamond crystal (or plate or film) which can be of a single composition or composed of layers of many compositions (called a “structure”). CVD diamond grown in this manner can show mechanical, tribological, and even electronic properties comparable to or exceeding those of natural diamond. See, for example, Y. Sato et al., “Synthesis of Diamond From the Vapor Phase”, The Properties of Natural and Synthetic Diamond, J. E. Field Academic Press, pp. 423-469 (1992). See also U.S. Pat. Nos. 4,940,015; 5,135,730; 5,387,310; 5,314,652; 4,905,227; and 4,767,608. Because of its ability for growth in terms of size and shape, CVD diamond can be used in a variety of applications. For a general analysis of differing applications, see M. A. Prelas et al., Handbook of Industrial Diamond and Diamond Films, Editors, Marcel Dekker, Inc., pp. 1023-1147 (1998).
Natural diamond is generally considered a good electrical insulator, however, if doped with appropriate impurities, it can be made into a good semiconductor as well. Because of this, there has been expanded research in terms of using monocrystalline diamond, both natural and synthetic, in a wide variety of electrical applications. Monocrystalline CVD diamond can be grown with sufficient control to achieve high performance electrical characteristics that are substantially greater than those found in natural high quality diamonds, as well as those found in other semi-conducting elements. These improved electrical characteristics include increased resistivity, increased breakdown voltage, increased carrier lifetime, increased electron and hole mobility, and increased charge collection distance. See PCT application WO 01/96633, the disclosure of which is incorporated herein by reference. In addition, it has been shown that the same improved electrical characteristics can be achieved with monocrystalline CVD diamond grown to a thickness of at least 2 mm. With this increased thickness, the grown CVD diamond can be utilized in additional applications that involve high pressures or temperatures (anvils) or involve cutting away layers (gemstone production). See PCT application WO 01/96634, the disclosure of which is incorporated herein by reference.
The properties of synthetic monocrystalline diamonds depend largely on the defects or impurities in the crystal. By controlling these factors, one can control not only the electrical properties, but also other properties of the diamond, including its optical and mechanical properties to name just a few (see M. A. Prelas et al., Handbook of Industrial Diamond and Diamond Films, Marcel Dekker, Inc., p. 20 (1998)). This realization has led to much research in terms of controlling the impurities during CVD growth. For example, it has been shown that the addition of boron to a synthetic monocrystalline or polycrystalline diamond makes it useful for constructing a semiconductor device, a strain gauge or other electrical device although monocrystalline diamond is to be preferred. See U.S. Pat. No. 5,635,258. See also, W. Ebert, et al., “Epitaxial Diamond Schottky Barrier Diode With On/Off Current Ratios in excess of 107 at High Temperatures”, Proceedings of IEDM, published by IEEE, pp. 419-422 (1994) and S. Sahli et al., “Piezoelectric Gauge Factor Measured at Different Fields and Temperatures”, Applications of Diamond Films and Related Materials, NIST Special Publications (885) pp. 95-98.
Thus, by doping CVD diamond, which is comprised of carbon isotopes, one can create the same devices, e.g., semiconductors, that have historically been created utilizing other materials than diamond that were more readily available. However, in using diamond instead of the other materials, all the vast properties of diamond can now be taken advantage of in the produced device.
With recent developments in the growth and fabrication of single crystal CVD diamond, there has been much excitement in the industry in regards to their overall utilization. However, efforts thus far have not produced the kind of quality diamonds that were originally intended or desired. While there remains a need for CVD diamond in the industry, there still needs to be a solution to the above-described shortcomings of the efforts to date.